FIG. 1 shows a typical lithium ion secondary battery. Referring to FIG. 1, a typical secondary battery 10 includes a can 11, an electrode assembly 12 accommodated in the can 11, a cap assembly 100 coupled to an upper opened portion of the can 11, a lead 200 for coupling an electrode terminal 120 of the cap assembly 100 to a protection circuit 220, and a washer 230 coupled to an upper portion of a cap plate 110 provided in the cap assembly 100.
In the lithium ion secondary battery 10, the can 11 is a container formed of metal substance and has a substantially hexahedral shape having an open upper portion. The can 11 is formed by a manufacturing method, such as a deep drawing method. The can 11 contains an electrolyte (not shown) and the electrode assembly 12 including a first electrode plate 13, a separator 14 and a second electrode plate 15. The electrode assembly 12 is inserted through the open upper portion of the can 11, and the open upper portion is sealed by the cap assembly 100.
The cap assembly 100 includes a flat type cap plate 110 having a size and shape corresponding to those of the open upper portion of the can 11. The cap plate 110 is formed at a center portion thereof with a terminal hole 111, into which an electrode terminal 120 is inserted, and a tube type gasket 130 for electrically insulating the electrode terminal 120 from the cap plate 110 is installed at an outer side of the electrode terminal 120 passing through the center portion of the cap plate 110. At a lower surface portion of the cap plate 110 near the terminal hole 111 of the cap plate 110, an insulation plate 140 is arranged. At a lower surface of the insulation plate 140, a terminal plate 150 is arranged to electrically couple to the electrode terminal 120.
The cap plate 110 further includes a safety vent 190 enabling gas to release when the inner pressure of the battery increases to reach up to more than a predetermined pressure. The safety vent 190 is formed at one side thereof with an electrolyte injection hole 112 in order to inject an electrolyte into the can 11. The electrolyte injection hole 112 is sealed by pressing a plug 160 therein.
At an upper portion of the cap plate 110, a washer 230 is provided for insulating a lead 200, connected to the electrode terminal 120 which is connected to the second electrode tap 17, from the cap plate 110 connected to the first electrode tap 16. The lead 200 electrically connected to the lead electrode terminal is located at an upper portion of the other side of the safety vent 190 and is electrically connected to the protection circuit 220.
However, since the washer 230 is located at the upper portion of the cap plate 110, the electrolyte injection hole 112 and the safety vent 190 are hidden by the washer 230. Accordingly, a leak of the electrolyte cannot be confirmed without removing the washer 230. Further, the worker must check the position of the safety vent 190 when assembling the battery and then the lead must be welded to the electrode terminal and the protection circuit toward the other side of the safety vent 190.